Audion amplifier



June I 17, 1930.

L. M. HULL AUDION AMPLIFIER Filed June 29, 1525 Patented June 17,1930

roams, s'EY AUDION AMPLIFIER 7 Application filed. Juneae, 1925.'SeriaI'JNo. 40,435.

I This invention relates to audion amplifiers and particularly toamplifier networks arranged to control or to eliminate retroactivecurrents through the'grid p'late capacity.

It has been proposed to construct an amplifier network as an alternatingcurrent l/Vheatstone bridge in which the input and.

the output circuits of theamplifier stage com- I prise conjugatearmswhich are electrically iisola'ted so far as alternating currents areconcerned, i 1 7 An object of'this invention is to provide an audionamplifying stage in which retroactiolnfrom output circuit uponinputcircuit P be made to vary in predetermined fashion with the frequency,may be made substan tially independent of frequlency or may besuppressed over a'wide rang of frequencies.

Specifically an object of the invention is to provide an audion circuitin whichinput and output circuits are made conjugate arms of analternating current Wheatstone bridge which has three substantially purecapacity balancing arms and a fourth" balancing arm comprising acomposite impedance the ef fective Capacity of which is a function Q-lnated entirely and the" balance may be 'ob-.

frequency,

Specific embodiments of my invention are 7 illustrated intheaccompanying drawings in ---30' which v I Fig. 1 is a circuitdiagranr of an audion ampl fylng stage constructed 1n accordance with myinvention; A a I Fig. 2 is a bridge diagram of theimpedance .elementsofthe alternating current network Fig. 3 is a circuit diagram of an audiondetector which due to the so-called regenerative effect also acts as aradio frequency amplifier; and V 7 Fig. 4 is a diagram of a cascadeamplifier. As will be seenfrom Fig. 1, the output cir:

cuit of the stage is arranged in the usualman' ner, and is there shownas comprls ng a coil L which may be the primary of 'a couplingtransformer for the succeeding staga' The input circuit is shown as animpedance L of the composite arm,

cuit is not connected across the input terminals G, F of the tube T, butis connected across the terminals G, 2, which terminals I designate asthe input terminals of the amplifying stage. Thepath from the lowerinput terminal 2 of the stage to thelower tube 7 terminal F is providedbya condenser C shunted by'an impedance L This path 21? forms one arm ofthe alternating current Wheatstone bridge and will be designated as thecomposite arm to distinguish from-the three capacity arms Ca; C and Ggwhich comprise, respectively, the inherent grid-plateca pacityofthetube,an external capacityj'connected between the plate P andfthe lower stageterminal 2, and the capacity between the grid and filament terminals. Asindicated in 1 Q IC LEWIS M. Humon Boonron; new JERSEY, nssienon TORania EREQ ENGYLABORAQ rnoonronnrnn, or BOONTON, new JERSEY,aconraonn'rron on N W JER- Fig. 1', the capacity C rbetwe'enthe grid andI filament terminals may be composed of the inherent tube'capacity G'plus anyexternal capacity G which is shunted across the tubecapacitv.Theexternal' capacity maybe variable. if desired, for purposesof adjustmerit, but this control isnot essential. In fact, the external capacityC may be elimitainedbya proper choice of the constants of the other armsof the bridge. The plate battery may be shunt'edby a'largecapacity CB toprovide a by-pass for the alternating cur-' rent... i

Thegnetwork ofFigl' theiform of a. I

Vl heatstone bridge as will be evident" from an inspection of Fig.2. Itis -'apparen.t that theconditionqforbalanceis given for allfre qquencies greater than'the resonant frequency where C is the-eifectivecapacity of the con l posite arm 11 G v U The arms ,C and G; are purecapacity arms hencetheir ratio in equation (1) is invariant frequency; Ccan be .made a pure capacity by properly biasing the grid of the audion,and the efiective capacity of the composite arm may be substantiallyindependent of the frequency or it may be made to vary with thefrequency, as desired. It is shown both by experiment and by elementarycircuit theory, that for all frequencies substantially greater than thevalue the effective capacity C of the arm F is given approximately bythe equation:

where e 21r frequency. Thus if the inductance of the choke coil L is sochosen that 1ts inductive reactance (0L is large compared with thecapacitive reactance of the capacity over a given range of frequencie's,the effect ve capacity C' Wlll be closelyequal to C and Wlll bepractically 1ndependent of frequency over this range. Ex-

plicity, suppose that in a given range of frequencies L 1s made so largethat L is greater than a i quencies." For instance if I wish to suppress retroactive currents by a complete bridge balance over the band ofwave lengths 200 to 1000 meters FQAZ 10 to 1.88 X 10 with a givenaudion, I-may make C of the order of magnitude 200 micromicrofarads. Lequal to or greater than 1. L henries, select C and C, so that Equation(1) is satisfied, using the value 200 10- for C and the bridge circuitis completely balanced, to an accuracy of one part in one thousand, withconsequent suppression of retroactive currents, over this specified waveband. I have found it convenient, when a balance substantiallyindependent of frequency over the whole range of radio frequencies isdesired, to useas L the secondary winding of an ordinary commercialaudio ffrequency transformer. C may comprise only the distributedcapacity of these windings or may include an external condenser as well.This use of an audio frequency secondary L is convenient when myinvention is applied to a circuit of the so called reflex type, in whichthe grid circuit of an audion is fed simultaneously from anaudio-frequency transformer and a radio-frequency transformer connectedin series. i

The useof the choke coil L across thearm C not only permits thereflexing of the balanced stage but has the additional and importantfunction of placing the direct current potential of the grid at apredetermined value for preventing extraneous noises and blocking in theaudion. So far as the con struction of a stage which is balancedindependent of frequency is concerned, the pres ence of the coil L isnot essential, but for the satisfactory reception of signals the chokeis of decidedvalue, particularly when several stages are cascaded.

The invention is not'limited however to amplifiers in which theinductance L and capacity C are so related that the effective capacity Cis practically invariant with frequency; By proper design of these partsit is possible to secure a substantially constant amplificationat allwave lengths even though the coupling transformers are of the type whichnormally result in a decrease in the amplification rate .with increasingwave length. At the higher wave lengths it is possible to operate withadegree of unbalance which would cause sustained oscillations orobjectionable reaction at the lower ranges. The unbalance may be sochosen that a predetermined regenerative effect supplements thenonregenerative amplification as the latter fallsoif at the lowerfrequencies.

From'Equation (2) it will be evident tha the effective capacity of thecomposite arm will vary rapidly with the frequency when theinductivereactance L w is greater than, but comparable'with, thecapacity reactance 1 0 50.)- The following specific example may becited: If L is made equalto 5 00 microhenries and C made 200micromicrofarads, the effective capacity C of the arm'to F is 177micromicrofarads at 200 meters, 110 micromicrofarads at 4:00 meters, andabove 596 meters the reactanceiofthis arm is no longer capacitilve butinchicti've. Thus if the arms (1, and C were chosen to provide a balanceat 200 meters, retroaction fromfthe plate circuit to the grid circuitwould increase progressively withthe wave length above this point. Atwavelengths above 596 meters noteven an approximate balance occurs,since one arm of the bridge isno longer capacitive, and if the outputcircuit is tuned or has a large induc which is the conventionalArmstrong regenerative detector having a plate variometer Ii whichcauses retroaction through the gridplate circuit with consequentregenerative increase in 'thetuned input circuit L1G. In accordance withmy invention a balancing network comprising C G and the composite arm L'C is added to'this circuit. At frequen'cles for whlch the cond1t1on ofbalance is satisfied no regeneration can occur for any value ofinductance ,L in the plate circuit Onthe other hand, when the bridge isun-, balanced, more or less regeneration occurs, depending upon theextent'of unbalance and upon the size of L It is possible so to adjustthe capacities of the bridge arms that a variation in the balancecondition with wave length occurs which produces maxi mum regenerationover a substantial band of wave lengthswithout altering the value of LThis effect is impossible in an ordinary regenerative circuit where, asis Well known, it is necessary to increase the plate circuit inductanceprogressively with increasing Wave length in order to maintain themaXimum regeneration.

7 As will'be apparent from Fig. 4;, stages such as illustrated inFig. 1may be cascaded in the usual manner when the inductance L of the firststage is coupled to an antenna circuit AG and the plate inductance Lofone stage and the input inductance Li of the succeeding stage take theform of the primary and the secondary, respectively, of a couplingtransformer. 1 a

The invention is not limited to the specific circuit details illustratedin the accompanying drawings nor to the particular constants hereinspecified, as the I arrangement of the bridge circuit and the relativevalues of the elements employed may be varied-within wide ranges withoutdepartingfrom my invention.

Obviously a complete balance'or the desired approximation thereto may beobtained by varying one or more of the quantities appearing in EquationSo far as I am aware it is'broadly new to provide an audion network inwhich the direct current potential of the grid is fixed with respect tothe filament, and in which the input and output circuits are connectedacross conjugateterminals of an alternating current Wheatstonebridge'having one or more balancing arms whose nnpedances arevar1- antwith frequency, and of which the remaining balancing arms aresubstantially pure capacities; My invention therefore contem- 7 platesall arrangements of this type whether ornot the bridge is balanced andhence nonregenerative over a substantial-range of frequencies, orwhether it is unbalanced and ience regenerative for a 130111011 011 therange of frequencies at which the amplifier is adapted to operate.

Iclaim: 1. In an audion amplifier network arranged' at a Wheatstonebridge which may] be balanced to controlretroactive currents, inputandoutput circuits of the amplifier stage forming the respective conjugatearms of the bridge, and four balancing arms, three of said armscomprising substantially pure 7 capacities and the fourth arm comprisinga composite impedance having an effective ca-I 'pacity which is afunctionof the frequency. 2. 1 In an amplifier, a three-electrodeaudion,

input and output circuits therefor adaptedjto formconjugate arms ofaVVheatstone bridge, three points of the said bridge being defined bythegrid,pla te and filament terminalsof said audion, and balancing armsconnecting theplat'e andthe filament terminals respectively with afourth bridge point, the impedanc'es comprising three of said arms'beingcapacities whose respective reactances at varying frequencies aresubstantially those of fined capacities, and theimpedance of the fourtharmbeing composite and having a reactance which, within'predeterminedlimits within the frequency range of theamplifier, varieswith frequencyat a rate substan tially different from that of a fixed capacity.

3. In an audion amplifier, an alternating current Wheatstone bridgenetwork comprising conjugate arms which form, respectively, the inputand output circuits of the amplifier stage,'threebalancing arms havingthe form of substantially pure capacitiesand a fourth arm comprising acomposite impedance havof the frequency. p I

4. An audion amplifier-stage the impedance mesh of which is arranged inthe form of an alternating current VVheatstone bridge of which theinputand output circuits of the,

amplifier stage form conjugate arms, the balancing arms'of said bridgecomprising sub! stantially pure capacity between grid and plate of theaudion tube, substantially pure substantially pure capacity and theother of said impedancescomprising a capacity shunt ed by aninductance,one terminalof saidining an effective capacity which is a function putcircuit'being connected to the grid of said 7 audion tube and the otherterminal ofsa'id input circuit being connected to a point'intermediatesaid two impedances in the plate circuit of said: audion tube. r

'5. In an audion a1nplifier,an

alternating f current Wheatstone bridge network comprising conjugatearms which form, respectively,"

the input and output circuits of the amplifier stage,- three balancingarms having the form of substantially pure capacities, and a fourthbalancing arm comprising a capacity shunted by an inductance, wherebythe effective capacity of said arm is a function of the frequency.

6. In an audion amplifier network ar ranged as a Wheatstone bridge Whichmay be balanced to control retroactlve currents, 1n-

put and output circuits of the amplifier stage forming the respectiveconjugate arms of the brldge, and four balanclng arms, three of saidarms comprising substantlally pure capacitiesand the fourth armcomprising aparallel inductance-capacity circuit of Which the reac'tanceof the inductive branch is comparable With, but greater than, thereactance of the capacitive branch, whereby the efiective'capacity ofthe composite impedance is a tunc- 'tion of the frequency. V p Intestimony whereof, I afiix my LE'WIS HULL.

signature.

